I’ve had several questions and Email about how I refinish the rough aluminum casting on the engine, transmission and rear drive and also some questions on how I got the carburetors to look shiny and new again.

NOTE: Refinishing is not rebuilding. Refinishing is restoring the surface of parts to “like new” condition. You are removing crud, stains, grime, dirt, grease, bugs, corrosion, tar, mud and old gasket sealer that have frequently been baked on for 10, 20, 30 or more years. It takes PATIENCE. It is applied Zen 🙂

Product Mentions

I make reference to specific products not to say these are the ONLY products that can work, but to be clear about what products I have used. YMMV. In particular, I have used AutoSol metal cleaning products and keep several of them handy. Here are links to the manufactures sites for you reference.

Refinishing Rough Cast Aluminum

There is a lot of cast aluminum in the R75/5: engine, air box halves, intake top cover, front cam cover, transmission, rear drive, lower fork tubes, front brake plate, and wheel hubs. Here is what I started with.

Front Brake Plate and Wheel Hub

Front Brake Cover

Air Box and Engine

Points Cover and Engine

Lower Engine

Lower Fork Sliders

Rear Drive with Baked on Stain from Gear Lube

Transmission

Transmission

Engine Cleaner

I start with engine cleaner to remove as much of the baked on grease and grime as I can. I use several kinds of nylon brushes, Scotch Brite pads and tooth brushes to get the junk off. I used engine cleaner on the entire bike before I brought it into the shop. I also used it to clean out the inside of the bell housing around the clutch and the exterior of the engine. I use a spay bottle of water to wash off the cleaner onto a thick layer of newspaper on the shop mat to soak up as much of it as I can.

Engine Degreaser to Clean Engine Bell Housing and Exterior

Brake Cleaner and Parts Washer

After I got the gunk off with engine cleaner, I put the smaller parts that fit into my parts washer and use the nylon brush and tooth brushes to get any remaining crud off. I use brake cleaner on stubborn stains and baked on tar. I also used it on exterior of the engine.

Cleaning Grunge Off Oil Pan

Scotch Brite and AutoSol Aluminum Cleaner

For the rougher castings found on the engine and transmission, I use Scotch Brite with the Autosol Aluminum cleaner. This usually removes stains from leaking battery acid, gear lube and oil.

Steel Wool

This comes in a variety of roughness. I use “00” rougher up to “0000” very fine. I use this with the Aluminum Cleaner to really renew the the pores in the cast aluminum. It quickly brings back a sparkle.

“000” Steel Wool with AutoSol Aluminum Cleaner on Transmission

(Left) Aluminum Cleaner with “000” Steel Wool; (Right) Parts Washed

Wet/Dry Sand Paper 600 – 2000 Grit For Scratch Removal

Some parts that were polished had scratches in them. For example, the shock covers. I used wet/dry sand paper starting with 600, then 1500 and finally 2000 grit to remove the scratches. Then I polished them with Griot’s polish starting with Polish #1 and ending with Polish #3. Then I finished up with Aluminum Polish.

Shock Covers: Shock Covers Before & After Sanding and Polishing

Gouge in Rim

Sanded and Polished Rim

Metal Polish, Aluminum Paste Polish, Aluminum Protective Oil

The last step is to polish the aluminum. For rougher parts, i use the AutoSol Metal Polish. For polished aluminum or fine grained castings, I use Autosol Aluminum Paste polish. I apply this with blue shop towels and remove the blackened residue with a clean section of the towel. I finish up using a microfiber towel. Finally, I spray the part with AutoSol Aluminum Protective Spray and let it sit for about 30 seconds and then lightly buff it with a clean microfiber towel.

Example Results

Here’s the results.

Polished Hubcaps

New Rubber, New Spokes & Polished Wheels

Refinished Front Forks Lowers

Powder Coated Right Control Perch with Polished Lever

Cleaned Oil Pan

Refinished Engine Case and Top Cover

Refinished Engine Case and Timing Cover

Refinished Oil Filter Cover & Lower Engine

Refinished Transmission

Refinished Transmission

Refinished Transmission

Refinished Rear Drive

Refinishing Bing Carburetors

To be clear, this covers cleaning and polishing the EXTERIOR of the carburetor body. I do NOT use abrasives, steel wool, etc. on the internal parts of the carburetor, only the exterior of the carburetor body.

Soak in Parts Cleaner Solution Overnight

I started with this (on the left) and ended up with this (on the right) after soaking the entire carburetor in parts cleaner overnight.

Before Soak, After Soak

Then, I use a tooth brush and my parts washer to remove any remaining dirt and crud off the outside of the carburetor.

Disassembly and Soak in Carburetor Cleaner

I use Chemtool B-12 carburetor cleaner. WARNING: This is very nasty stuff. It eats nitrile gloves. Use rubber kitchen gloves when handling it and keep it off your skin. Wear safety glasses when working with it.

Chemtool B-12 Varnish & Gunge Cleaner

I disassemble the carburetor and soak all the parts overnight in bins.

Soaking Carburetor Body Overnight

This is what the body looks like after I use a tooth brush with Chemtool B-12 to clean any remaining varnish and crud out of the creases and crevices of the carburetor body.

Carburetor Body After B12 Soak and Tooth Brushing

Polishing Exterior of Carburetor Body

Here are the basic tools I use AFTER I have soaked the carburetors in Chemtool B-12 carburetor cleaner overnight followed up with a tooth brush scrub with B-12.

Tools for Cleaning, Polishing Carburetor Body

Most of the work is done with “0000” steel wool and AutoSol Aluminum Cleaner. I spray the AutoSol cleaner and then use the steel wool to return a shine to the aluminum. In the hard to reach places. I use the the brass wire brush and brass Dremel tool to get into some of the curved and hard to reach creases in the body. I use a lot of blue shop towels to clean the blackened residue off as I go. Next, I use the AutoSol Metal Polish and a blue shop towel to bring the luster back to the aluminum. In hard to reach places, I put a brittle brush on the Dremel tool to polish. Again, I use a separate blue shop towel to buff and remove the black residue. I sometimes use the AutoSol Aluminum Paste polish. It works well on already polished surfaces but I find the liquid Metal Polish works better on the rougher cast aluminum found in the carburetor body. I polish up all the exterior metal hardware (linkages, levers, nuts, etc.) using the Metal Polish and-you guesed it-more blue shop towels. Here is the final result after several hours of applied elbow grease.

Polished Carburetor Body, Throttle Stop and Spring

Cleaned & Polished – Top View

Cleaned and Polished, Outside View

Cleaned and Polished, Inside View

Refinishing Chrome Headers

I have used AutoSol Bluing Remover and Semichrome paste with good results. You can read about the work I did to refinish the header pipes here:

Refinishing Faded Black Plastic

I got a comment from a reader that I should try “Forever Black” on faded black plastic to restore the sheen. I tried that on the handlebar switches and I think this is going to be my new “go to” for plastic restoration.

26 thoughts on “51 BMW 1973 R75/5 Refinishing Techniques”

Hi brook!
Thanks for the amazing break down of everything. Quick question. I just received an r75/6 motor without the cylinders on it. The engine block is caked with crud but the inside is surprisingly clean. How can I safely clean the outside without damaging the inside? And also, if any materials/liquid I.e. Water – chemicals get inside engine how would one clean that out? Thanks!

Thanks for stopping by and I’m pleased this material is helpful to you on your project.

If the cylinder studs are with the block, I’d cut a piece of 1/4 inch plywood to fit over the opening for the pistons and then tape the edges with masking tape. You can clean the block using engine cleaner and brushes, and clean with aluminum cleaner as I describe. I used a spray bottle to wash the engine cleaner off the engine so water didn’t get inside the engine.

If you are concerned there is debris inside the block where the crankshaft and rods are, you can you use brake cleaner to clean that out. If the oil pan is removed, any crud inside will drain out. Then, I would use an oil can to coat the connecting rod bearings and crankshaft bearings with oil so they will have some lubrication to prevent any rust and some available when you first start the engine.

I used the electric starter to pump up oil pressure until I saw oil coming from the rocker arms at the top of the heads. This helped fill all the internal passages with oil and provide some lubrication to bearings before I tried to start the engine.

Well, no. The way the hinges are mounted, you have to remove the two screws not covered by the frame tube and then rotate the hinges enough to get them off the hinge pins on the subframe. At least I haven’t found any other way.

On their web page, they list an Austin, TX PO Box and phone numbers. Why not give them a call to see if there are “distrubutors” or retail outlets that carry the product. I just order it from their web page and it shows up in a couple of days.

Thank you for posting all of this for everyone to see. I have had an R100/7 for almost a year now. It is in pretty bad condition though. So much grime all over the engine, rusted spokes on the wheels, etc. I am going to use this guide to do a refinish, and have the frame powdercoated.

Did you also refinish the cylinders? I am a little worried about those guys because I think it may be hard to get in between all of the fins.

I had the cylinders “walnut shell” blasted at a local shop. I emphasize “walnut shell” as glass beads can be a problem with small pieces of glass embedded in the aluminum and sand can erode the aluminum very quickly. Other options folks have used include soda blasting and vapor blasting.

Thanks for this! I am using Autosol Power Aluminium Cleaner and scrubbing this in using 3M sanding blocks (#80 so far with very positive results, and I am also going to try #150 grade). I then apply Autosol Aluminium Polish, but quite frankly the cleaner solution is working so well that the polish doesn’t really enhance the finish — though it might work to preserve the look. Looking forward to giving the back of the brake rotor carriers a good clean up too. Cheers ~ kaleuclint

Well, thanks for coming by and visiting. I’m glad this material is helping you get another airhead back on the road. If you have questions as your project progresses, feel free to post them on the appropriate write-up page and I and/or others will help you out.

I like cleaning the motor, transmission and the rest of cast parts before dismantling the entire bike. With the tank and seat off it’s no big deal to lay it over to each side to get beneath it. I use GUNK to get the oily residue and dirt off then an acidic aluminum cleaner/brightener such as Nice & Easy or refrigeration coil cleaner sold at Lowes. That stuff is applied diluted to a wet surface then brushed about until the foaming stops which signals it’s time to be rinsed off. Those make the cast parts look like new with little effort and in record time. The remaining cleaner can be diluted more for use on smooth aluminum to remove their lighter oxidation so less polishing is required to shine those up. I’m currently using the coil cleaner bought from Lowes since a gallon of that is cheap and also use it to clean the coils on my HVAC outside and indoor coils plus my vehicles with aluminum finned radiators.

Hey Brook,
Great blog. I have a R75/5 that I am about to do a top end job and some other restoration work. I bought my BMW in Dusseldorf Germany in 1977 and rode it around Europe for 4 months before shipping it hoe. The dude I bought it from over there rode this bike from Germany to India and back. I have had it coast to coast and up to Alaska. I semi-retired the bike when one of my girlfriends freaked out on me about safety and bla blah blah. She’s out of the picture now…hahaha

I am impressed with the before and after photos of the aluminum castings and metal work.

Question: Is there some magic product to apply to te brightened metalwork to preserve the restored finish? I hate to do all that work and then have the aluminum discolor again in a short time.

Also, do you have any recommendation on parts suppliers for top end rebuild, front fork rubber covers, miscellaneous supplies?

I have used Autosol Aluminum Protectant spray. It seems to reduce the dulling of the aluminum finish. That said, it’s the nature of metal to oxidize and collect crud if a bike is ridden. It’s okay 🙂 If it really gets bad, it’s time to clean it up again.

If you wish to do the head work yourself, Ted, Anton and Tom Cutter at rubber chicken racing garage (rubberchickenracinggarage.com) can provide parts. Note machining valve seats does require knowledge about airhead requirements that most auto machine shops lack. Most BMW motorcycle dealerships sell all airhead parts that are still available from BMW as well. I like to use MAX BMW parts fiche to find what I need (http://www.maxbmwmotorcycles.com/fiche/PartsFiche.aspx) and order from them as well.

Hi Brook,
thoroughly enjoyed this piece and has given me the impetus to give my R80 the elbow grease it needs. Unfortunately some of the products you guys use are not available here on the emerald isle and I end up looking for an equivalent which sometimes gives interesting results.
Is there a surefire method for cleaning oxidation on the fins, or rather where should I start.
Dave
Greeting from sunny Wexford, Ireland

Thank you for dropping by and reading this material. I’ve been able to order directly from AutoSol. They are in Germany, so they may/should be able to ship to Ireland. Or, you could just plan a nice little trip to visit them, 🙂

I have removed the cylinders, and took them to be blasted at a local motorcycle shop to clean in between the fins. Walnut shells or “soda” are good media to ask them to use. It was not very expensive here in the US. I have not tried doing this by hand.

A search with Google, “tool to clean motorcycle cylinder fins”, shows a number of approaches to try with the cylinders on the engine. There are some brass wire wheels with fine wires (no info on who makes these), scotch-brite pads wrapped around a flat paint stir stick and scrubbing between the fins. I hear a very nice finish is achieved using “vapor blasting” which is very popular in the UK, but only recently showing up here in the US. I suspect you can find a shop with a vapor blaster that would do the work with the cylinders off the engine.

Amazingly great information that you provide. As a mechanic for over years it is still helpful to hear other approaches to maintaining our BMW’s. My 1980 R100rs is kept clean, and I would love to have it shine but I prefer to have it ridden but it is a good trade off. Safe riding

I much admire all of your publishing in the public domain. I am based in South Africa and find it difficult to find such openness in what other people have found successful. If you don’t mind I would like to deal with you directly with some of my queries. I am sure that you have responded to a few of my queries related to my restoring a R75/6
Stan R